1. Field of Invention
This invention relates to pumps which are designed for pumping high pressure, high temperature, demineralized water (product water), such as used in boiling and pressurized water nuclear reactors. These pumps have a plurality of heat exchangers to cool the shaft seals and other components and this invention is specifically directed to the improvement of these heat exchangers to solve the problem of shaft and cover thermal cracking from the effects of seal purge water and product water mix and thus prolong the operating life of the pump assembly.
2. Prior Art
FIG. 1 shows a prior art pump assembly and FIG. 2 shows an impeller and hydrostatic bearing in the pump assembly of FIG.1. FIG. 3 is a schematic illustration of the working relationship of the heat exchangers in the pump assembly of FIG.1.
More specifically, FIG. 1 shows a pump assembly 10 which includes a pump housing 11, one outlet port 12 and a motor 13 connected to one end of a shaft 14, which extends through a bore 15 in a pump cover 16, for driving impeller 17 as shown in FIG.2. The pump impeller 17 with its inlet port 18 and outlet ports 20 is shown connected to a cylindrical journal 21 and surrounded by a hydrostatic bearing 22 and pumps product water, represented by arrows 23, at high pressure through outlets 20. This pump assembly 10 is described in detail in the U.S. Pat. No. 4,775,293 of Boster to which reference may be made.
FIG.3 shows the motor 13 attached to the shaft 14, shown as a center line, to drive the impeller 17. FIG.3 also shows three heat exchange areas 24, 25 and 26; the latter being the cover bore 15 incorporating this invention as an improvement in the entire pump assembly, which improvement will be described last so that the problem solved by this invention may be discussed at length.
Thus, the first heat exchanger area 24 is shown within a driver mount 27 surrounding a stuffing box 28 in which component cooling water, represented by arrows 30, is passed through a heat exchanger 31 surrounding the stuffing box 28 and then down through a plurality of vertical holes 32 located near bore 15 in cover 16. Thereafter the component cooling water 30 is returned through the heat exchanger 31 and out through the driver mount 27 opening.
Seal purge water, represented by arrows 33, is injected into the stuffing box 28 where it is circulated by an auxiliary impeller 34 driven by the shaft 14 to circulate through an external heat exchanger 35. Heat exchanger 35 comprises helically formed tubes, represented by staggered lines 36, located in a water jacket 37 which is also cooled by component cooling water 30. Excess seal purge water 33 is also directed along the shaft 14, through a bore 15 in the cover 16, and into a mixing region 38 located where shaft 14 exits bore 15. Product water 23 is circulated from the outlet 12 through the hydrostatic bearing 22 into the mixing region 38.
The seal purge water 33 in the area of the auxiliary impeller 34 also cools a multi-stage mechanical seal assembly comprising mechanical seals 40 and 41 which prevent liquid from entering the motor 13 or the adjacent environment. The lower mechanical seal 40 is subjected to the full pressure of the seal purge water 33 which also flows, as a controlled bleed off, through a staged pressure reducing means, represented by the staggered lines 33a, so that the pressure in area 42 between the two mechanical seals is reduced by one-half. The second mechanical seal 41 is subjected to the reduced pressure in area 42 which is bled off through a second stage pressure reducing means, represented by staggered lines 33b, so that the pressure in area 43 between the motor 13 and the second mechanical seal 41 is reduced to almost zero where the seal purge water 33 is then directed out the stuffing box 28 as shown at 33c. The area containing the mechanical seals 40 and 41 is called a "seal cavity" and includes a "seal stage area". The mechanical seals 40 and 41 and the stage pressure reducing means themselves are fully described in the U.S. Pat. No. 4,586, 719 of Marsi et al and in the U.S. patent application, Ser. No. 07/488,238, filed Mar. 1, 1990, by Marsi entitled "Mechanical Seal" so no further details of the mechanical seal assembly need to be described.
The second heat exchanger area 25 containing the shaft driven auxiliary impeller 34 and the external heat exchanger 35 serves to maintain the seal purge water 33 at a low temperature so that the mechanical seals 40 and 41 are protected against overheating and purged of particulate matter.
As an alternative to the auxiliary impeller type heat exchanger, the heat exchanger may comprise a multi-flow, multi-path rotating baffle type heat exchanger which surrounds the shaft 14 and, like the auxiliary impeller 34, is located between the impeller 17 and the mechanical seal assembly. This heat exchanger 25 is also subjected to excess purge water 33, ie, more than necessary to purge the mechanical seals which is directed along the shaft 14 through the bore 15 in the cover 16. This rotating baffle type heat exchanger is fully described in the U.S. Pat. No. 4,775,298, supra, so no further details concerning the function and operation of this type of heat exchanger need to be described further. See also U.S. Pat. No. 4,005,747 of Ball.
These heat exchangers, whether of the auxiliary impeller type or the rotating baffle type serve to prevent heating and damage to the mechanical seals 40 and 41 if the flow of seal purge water 33 were to cease. This is represented by arrows 23a showing product water 23 flowing upwardly along shaft 14 and into the external heat exchanger 35 where the seal controlled bleed off water is cooled. This is also fully explained in the two patents referenced above.
It is to be understood also that either of these heat exchangers may be used in connection with this invention although the invention is disclosed in connection with the rotating baffle type heat exchanger.
The third heat exchanger area 26 is in the region in which the shaft 14 passes through the bore 15 and is near the hydrostatic bearing 22 where the flow of excess seal purge water 33 enters the mixing region 38 and mixes with the product water 23. As best seen in FIG.2, the mixing region 38 is defined by an annulus 43 below the cover 16 where the shaft 14 is within the hydrostatic bearing. Hydrodynamically induced turbulences and non-uniform flow paths between the product water 23 in an area 44, adjacent to the top of the hydrostatic bearing 22, and the product water 23 in the mixing region 38 causes the product water 23 to enter and mix with the seal purge water 33 in the mixing region 38 and impinge on the shaft 14 and cover 16 where the shaft 14 exits the bore 15. The mixture then exits to the low pressure zone of the impeller 17 through openings 45.
However, as excess seal purge water 33 flows along the pump shaft 14 and through the bore 15, very little heat-up occurs. Thus, temperature of the seal purge water 33 is substantially the same as when it entered the seal cavity.
Since the mixing region 38 contains high temperature water from the hydrostatic bearing, mixing of the hot and cold water will occur in this area. This mixing results in localized hot and cold flow regimes alternately impinging on the shaft 14 and cover 16 in the mixing region 38. The cyclical heating and cooling induces surface thermal stresses both in the cover bore 15 and on the surface of the shaft 14 which, over a period of time, can result in cracking. These cracking areas are represented by dashed lines 46 and 47 in the shaft and cover, as shown in FIG.2. Some of the cracks not only penetrate deeply, but may be oriented so they can lead to a structural failure of either or both the cover and the shaft.
Extensive calculations have been made to identify mechanisms of crack initiation and propagation as well as to develop means for mitigating cracking tendencies. The calculations simulate the mixing phenomenon by hypothesizing pulsations at various frequencies and amplitudes. The results decribe crack depths as a function of total operating time. FIG. 4 shows such a calculated result compared against field data obtained from operating plants worldwide. The fact that there is good agreement between theory and actual observations leads to the belief that the theory is sound and that counter measures against cracking can be established.
It is clear that the root cause for crack initation is the high temperature difference (.DELTA.T) at the exit of the thermal barrier between the seal purge water 33 and the product water 23. Parametric studies have shown that this .DELTA.T cannot be reduced significantly by changing operating conditions. For example, increasing seal purge water at the point of injection temperature reduces the .DELTA.T only by the amount of the inlet temperature increase. Since cracking cannot be prevented unless .DELTA.T is reduced to below about 100 degrees F., and the normal .DELTA.T is about 330 degrees F. (this number has been obtained by detailed calculations), this injection temperature has to be increased by over 200 degrees F. This is not acceptable because of seal cavity temperature limitations. Also, changing the flow of seal purge water 33 is not totally effective. FIG.5 shows that decreasing net downflow to 0.5 gpm reduces cracking tendency, but does not eliminate it. Completely eliminating seal purge water 33 will eliminate cracking at the bottom of the cover 16, but since controlled bleed-off flow for the mechanical seals 40 and 41 has to be from product water 23, mixing will occur at the top of the cover bore 15 and cause cracking there. Calculations and field observation have confirmed this.
As a result of these studies, it has been concluded that the .DELTA.T itself has to be decreased. Since the temperature of the seal purge water 33 has to be maintained below about 150 degrees F., it is necessary to heat the down flowing seal purge water 33 after it leaves the seal cavity area and before mixing with the product water 23. This patent application covers a concept of purge water heating as mentioned above.